Preserving personal care products from microbial degradation is quite challenging. Most topical cosmetics and dermatological products in the form of creams, lotions, gels, shampoos, body-washes and face-washes contain significant amount of water. This provides a very hospitable environment for the microbial growth. In addition to water, other cosmetic ingredients can also be a good source of nutrients to microbes. Another pertinent point to be reckoned here is that the shelf-life of the personal care products and the period after opening the container by the consumer is quite long compared to pharmaceutical products or food products. Unlike pharmaceutical products, cosmetics products are neither sterilized nor packed in hermetic conditions. Thus, the requirement for preservation of the personal care products is, indeed, quite challenging. This is further compounded by the limited choice of antimicrobials since the available approved antimicrobials are very few and those which have good antimicrobial activity are quite toxic. Consumers want products meant for topical applications to be free from toxic antimicrobials that are used as preservatives. Very effective antimicrobials that are used currently, are implicated in serious toxicity issues to human as well as to environment. For example, parabens are implicated in disrupting endocrine system, ultimately linked to breast cancer [(Pharmacology & Toxicology (Vol. 86(3), pp 110-13, March 2000, Toxicology and Applied Pharmacology (Vol. 153(1), pp. 12-19 (November 1998), Journal of Veterinary Medical Science (Vol. 64(3), pp. 227-35 (March 2002, Journal of Applied Toxicology, 24 (3): 167-176, (2004)]. Formaldehyde is classified as Category 3 CMR (carcinogenic, mutagenic and reproductive toxic) and hence all formaldehyde releasers are under the cloud. This class includes the work-horse preservatives such as DMDM hydantoin, diazolidinyl urea, imidazolidinyl urea and Quaternary 15.
Another class of very effective antimicrobials is ‘isothiazolinones’. Methyl and chloromethyl isothiazolinones have been used in personal care but these are reported to be neurotoxic and skin sensitizers (Journal of Neuroscience 22 (17): 7408-7416. The Lancet, Volume 333, Issue 8633, Pages 314-316 (1989). Chloromethyl isothiazolinone (generally abbreviated as CIT) is far more toxic and most of the leading personal care products manufacturers have stopped using it.
Halogenated antimicrobials have their own share of toxicity issues. For example, Triclosan, once a popular antimicrobial for hand sanitizers, is being phased out due to its toxicity. It has been implicated in eco-toxicity issues (algae, dolphins). It is reported to be an endocrine disruptor (thyroid function) and is reported to impair cardiac and skeletal muscles. There seems to be special concern for children who are at higher risk of allergies and the immune systems (Toxicological Sciences, 2009, 107 (1): 56-64, Reproductive Toxicology, April 2009, 27(2): 177-185). Companies such as Johnson and Johnson, Proctor & Gamble and Reckitt Benckiser have removed it from their products. Triclosan's ecotoxicity is such a big concern that its usage has been completely banned by the state of Minnesota in the USA. Iodopropynyl butyl carbamate, another halogenated antimicrobial, is a contact allergen (American Journal of contact dermatitis 13(2), 77-79 (2002). The presence of iodine in the molecular structure gets it implicated in Goiter and malfunctioning thyroid gland. It is not allowed in Japan and in European Union (EU) and generally, elsewhere, it is allowed only up to 0.02% in leave-on products. Similarly, EU permits usage of methyl dibromo glutaronitrile only up to 0.1% and that too in only rinse-off products. Another brominated molecule is Bronopol, very widely used once upon a time, is banned today in countries like Canada for its application in cosmetics. It is involved in allergic reactions as well as generation of N-nitroso amines that are known to be carcinogenic. The quaternary ammonium compounds (examples are cetyl pyridinium chloride, benzethonium chloride, benzalkonium chloride) exhibit good antimicrobial activity but their utility in personal care industry is limited due to incompatibility with anionic surface active agents. The cationic antimicrobials are completely neutralized by the anionic surface active agents that are present in large excess in home and personal care formulation (8 to 20%).
Thus, most of the antimicrobials with phenolic nature or containing halogen (chlorine, bromine, or iodine) are toxic. Also, the big class of ‘formaldehyde releasing antimicrobials’ is being phased out due to highly toxic nature of formaldehyde. Examples of this class are DMDM hydantoin, imidazolidinyl urea and diazolidinyl urea.
To avoid the above mentioned toxic antimicrobials, the industry did come up with alternatives antimicrobial preservatives that are largely based on organic acids such as sorbic acid, benzoic acid, dehydroacetic acid, and alcohols such as phenoxyethanol, benzyl alcohol (Cosmetics Directive Annex VI). However, the biggest disadvantage with all organic acids is that they are effective in very narrow pH range of acidic side typically, 3.0 to 6.0. They tend to lose their efficacy fast as the pH approaches the neutral point or above 7.0. Alcoholic antimicrobials like phenoxyethanol and benzyl alcohol are liquids and exert solvent action in formulations and they do have their share of problems in terms of drop in the viscosity of the formulations. In September 2012, French Agency ANSM (Agence nationale de sécurité des médicaments et des produits de santé) which rose concerns about the use of Phenoxyethanol as preservatives for baby care products due to inadequate safety data and demanded that the upper limit for dose be lowered to 0.4% from 1.0% for baby care products meant for children under the age of 3. In 2016 Scientific Community of Consumer Safety (SCCS) finally declared it to be safe up to 1.0% level for cosmetics. However, the concern remains with the manufacturers of personal care products because of Phenoxyethanol's belonging to the dangerous glycol ether family (methyl cellosolve or ethyl cellosolve, phenoxy ethanol is phenyl cellosolve). Phenoxyethanol is also reported to depress the central nervous system (Schmuck G, Steffens W, Bomhard E (July 2000). “2-Phenoxyethanol: a neurotoxicant?” Archives of Toxicology. 74 (4-5): 281-3) and it may cause vomiting and diarrhea, which can lead to dehydration in infants. There are some baby care products that mention on the label about being free from phenoxyethanol. (Colgate's baby care brand Tom's of Main, moisturizing lotion).
Benzyl alcohol, another antimicrobial, suffers from disadvantage of being allergen causing allergic contact dermatitis reactions (E. J. Curry and E. M. Warshaw, Dermatitis, 2005; 16 (4): 203-208) and strong benzaldehyde like aroma puts a limitation on its deployment as antimicrobial across the personal care products.
So in view of the limitations, restrictions and concerns cited above for the majority of antimicrobials, there remains an urgent need to develop a broad spectrum, organoleptically acceptable antimicrobial, non-toxic (non-phenolic, non-halogenated, non-formaldehyde releaser) and compatible with anionic surfactants which are the mainstay of personal care and for home care formulations.